One known type of valve drive apparatus, installed in double overhead camshaft engines, uses a timing belt to couple either one of two overhead camshafts, interconnected to each other with a gearing, to an engine crankshaft in order to transmit rotation of the engine crankshaft to the two overhead camshafts so as to open and close intake and exhaust valves at a desired timing.
To protect the timing belt against damage, the engine is provided with a front cover attached thereto to cover and hide the timing belt. This front cover is sealingly secured to the front end of the engine with a seal plate interposed therebetween. The seal plate is configured so as to surround a camshaft pulley attached to the overhead camshaft coupled to the engine crankshaft.
Although such a double overhead camshaft engine always must be as compact as possible and has to be provided with high performance efficiency, the overall length of the engine is increased in order to provide sufficient space, in an axial direction of the camshaft, for the sealing plate to be interposed between the front cover and the front end of a cylinder head of the engine and, in some cases, the front end of a head cover of the engine. Otherwise, if the wall thickness of the cylinder head or the head cover is decreased in an axial direction of the camshaft, in order to provide sufficient space while attempting not to increase the overall length of the engine, supporting structure for the valve drive apparatus may be subjected to a decreased structural rigidity.